The present invention relates to a virtual addressing technique for converting logical addresses into physical addresses representative of a plurality of addressable storage locations in a data processing system and more particularly to such a technique which may be suitable for the case where the bit length of a virtual or logical address is shorter than that of a physical address as in a minicomputer.
In so-called minicomputers in which a virtual or logical address is written with 16 bits, only 2.sup.16 (=64K) words or locations can be addressed directly. In recent minicomputers, the 16-bit virtual address is converted into, for example, a 20-bit physical address through a mapping mechanism so as to obtain a physical space of about 1M words. Such a computer is generally called "Mega Mini" (referring to a minicomputer having a memory of mega byte capacity). Though the number of physical addresses can be thus increased, there is a restriction that the number of virtual addresses can be no greater than 64K. Examples of the conversion of virtual addresses into physical addresses and details of the mapping mechanism are shown in various references. For example, one can make reference to IBM GA34-0021-0 SERIES/1 Model 5 4955 Processor and Processor Features Description, published on November, 1976, especially, pages 2-17 to 2-24 (disclosing Effective Address Generation) and pages 9-3 (describing Relocation Addressing and showing an Address Translation Example in FIG. 9-1); and the Gray et al U.S. Pat. No. 3,854,126 issued on Dec. 10, 1974 and entitled "CIRCUIT FOR CONVERTING VIRTUAL ADDRESSES INTO PHYSICAL ADDRESSES" which discloses the need for address expansion and the conversion of a 16-bit virtual address into a 18-bit physical address.
In operating systems (or executive programs) or control applications, a space larger than 64K words must be sometimes treated as a common area between tasks on the basis of one program. In such a case, it was necessary to change the setting of the map. This is inefficient as well as laborious in that the programming must be made taking account of which map is now used.